Can someone answer these questions for me:
1. Is the Vita fully intergrated into the PS Suite?
It's the other way round. Playstation Suite is integrated into Vita OS, like how WebKit, .NET sit on top of Mac OSX, Windows, Linux, etc.
2. Can all PS Suite apps work in Vita and vice versa?
Kaz Hirai created PS Suite to unify and extend Playstation. Major PS Suite apps should work on all Playstation certified hardware. This includes Vita, PS3, Tablet S, Tablet P, some Xperia phones. Their next TV may be PS (Suite) certified too. They are willing/keen to add third party devices under the PS Suite umbrella also.
3. Is PS Suite the base OS for all Sony Ericson phones?
No. It's the application run-time above the base OS. Based on Mono, it includes GUI building tools, C# language run-time, and Playstation oriented class libraries. It's like Microsoft's .NET framework but for games.
4. Can Sony mobile users fully interact with Vita owners?
What do you mean by interact ? Vita has PSN, FaceBook, FourSquare, Twitter, Skype and Flickr apps. The quality of these apps will dictate how well Vita users interact with the rest of the world. Not sure if these apps are running on top of PS Suite (They should if Sony and partners want to save resources).
If you're asking about a special mechanism for Sony phones to talk to Vita exclusively, there is none. Once the app is on PS Suite, it should run on all PS Suite hardware, not just Vita or Sony mobile devices.
Don't know how Sony intend to integrate WebKit and Mono though, since they are both app frameworks specialized for different experiences. Both Vita and PS3 have a WebKit run-time.